For desulfurizing a power plant flue gas, there has been known a method of using lime (CaO) or limestone (CaCO3). The use of these calcium compounds is advantageous in terms of cost but they entail a large amount of contaminated water as well as a low-value plaster as a product.
Amine compounds such as triethanolamine, monoethanolamine, diethanolamine, etc. also have been used as a SO2 absorbent. However, a high regeneration temperature caused by a strong chemical bond between the amine and the sulfur dioxide results in irreversible decomposition of the amine. Furthermore, due to the vapor pressure of amine absorbents, regenerated SO2 gas is frequently contaminated by the amine absorbents. Recently, as disclosed in U.S. Pat. No. 6,849,774 B2, U.S. Pat. No. 6,623,659 B2, and U.S. Patent Application Publication No. 2008/0146849 A1, methods of using an ionic liquid, which is non-volatile and has high thermal and chemical stability, have been attempted to overcome the disadvantages in the use of the conventional absorbents.
According to Angew. Chem., Int. Ed. (2004, 43, 2415-2417), 1,1,3,3-tetramethylguanidinium lactate ([TMG]L) could absorb 0.978 mol of SO2 per mol of ionic liquid. Similarly, according to Korean Patent Application No. 10-2007-0034777, [BMIm]Cl could capture 1.68 mol per mol IL, while [EMIm]EtSO4 could absorb 0.92 mol SO2 per mol of ionic liquid.
Recently, in Energy Environ. Sci., 4, 1802-1806 (2011) and Chem. Commun. 2633-2635 (2012), as a method for increasing SO2 absorption capacity, ionic liquids having an ether functional cation and a tetrazolate anion are reported, respectively. These ionic liquids could capture more than 5 mol of SO2 per mol of ionic liquid due to the presence of Lewis basic sites which could interact with Lewis acidic SO2 
Meanwhile, flue gas contains water in the amount of 5 to 15% ionic liquid. In the presence of water, SO2 exists as an hydrated form, SO2.H2O. The hydrated SO2 is a strong acid SO2.H2O as described below.SO2.H2O⇄HSO3−+H+ pKa=1.8
SO2.H2O is much different from SO2 in terms of chemical property. That is, SO2.H2 is not a Lewis acid but Bransted acid. Therefore, when SO2.H2O solution is treated with amine, an ammonium bisulfite salt is produced. If the basicity of amine is high, regeneration of an absorbent is very difficult. Actually, J. Haz. Mat. 229-230 (2012) 398 describes a method for absorbing SO2 by using an amino acid type absorbent dissovled in water and regenerating the absorbent. In the present disclosure, regeneration of the absorbent occurs at the temperature of 150° C. in which all water is evaporated together, thereby increasing regeneration energy.